Hard turning machines are becoming more common on a macro-scale, replacing traditional turning machine tools. At the micro-scale, hard turning machine tools employ methods for dampening vibration and increasing stiffness using conventional machine tool topologies. However, hard turning of miniature parts or components presents unique problems for micro-scale machines.
As a nonlimiting example, miniature bearings have been used for a wide range of applications from dental spindles to gyroscopes in missiles. Such bearings conventionally are made of hardened steel and produced on the same machines as large bearings, then finished on a grinder. This process is time consuming, and produces low yields of bearings with inconsistent life expectancies.
More recently, hard-turning has been shown to be a viable alternative. Hard-turning has the advantage of not requiring custom tooling for every part and creates a residual stress pattern at and below the surface favorable to bearing life. While miniaturizing hard turning machines could eliminate the need for grinding small bearings to a finish, problems arise as the size of the bearing components made on these machines reduces and the machine components shrink. For example, the dimensional accuracy requirement of a bearing feature is typically relative to its size. As the bearing becomes smaller the tolerances become tighter. The tolerances on these miniature bearings can easily reach 1 μm, pushing or exceeding the limits of traditional machines. Therefore, miniature bearings made on such machines may require selective assembly to meet tolerances. As a result, this can lead to low yields, particularly with small batch sizes common to miniature bearings.
Some previous designs using hard turning of miniature bearings using a micro-scale machine tool (mMT) have achieved good accuracy and surface finishes, but their processes have not been robust. For more stable cutting conditions for miniature bearings and other miniature parts, it is desired to provide a more rigid machine tool that improves both accuracy and surface roughness, while limiting problems such as chatter.